Links among Slope Morphology, Canyon Types and Tectonics on Passive and Active Margins in the Northernmost South China Sea

We examine slope profile types and variations in slope gradient and slope refief with depth for both passive and active margins in the northern most South China Sea. The passive South China margin is characterized by an exponential slope profile, mainly associated with clustered slope-confined canyons. The active Taiwan margin shows a linear-like shape with great variations along the lower slope. Fewer canyons occur on the Taiwan margin, and hence the influence of canyon incision on slope morphology is relatively less significant. Quantitative analyses of slope curvature, slope gradient and square root of relief variance are useful statistical parameters to explain characteristics and variability of morphology of the slope of the South China margin, but not for the Kaoping slope on the Taiwan side. On the active Taiwan margin, tectonic activities are dominant over sediment deposition and surface erosion, producing a slope profile quite different from those of passive margins of the Middle Atlantic, KwaZulu-Natal, South Africa where failure on slope and accompanying canyon incision are the dominant processes shaping the slope morphology.

Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa

Both the shelf-edge trajectories and slope morphology are indicative of deep-water sedimentation, but previous studies are relatively independent from each other in the two dimensions. An integrated investigation can enhance the understanding of deep-water sedimentary systems and enrich reservoir prediction methods. Based on the bathymetry data and seismic data published, this study identified ten slope areas at the continental margin of East Africa and classified the clinoforms into three types: concave-up, sigmoidal and planar. Combined with the distribution of main modern rivers in East Africa, nine modern source-to-sink systems were identified and the catchment area is positively correlated with the size of the shelf-edge delta. It is found that the slope morphology of East Africa is closely related to the geological setting, sediment supply and sediment transport pathway in submarine canyon of passive continental margin. When the sediment supply is stable, the concave-up slopes are dominated by the river-associated and shelf-incising canyons and the sigmoidal slopes are determined by the headless canyons. There exists a strong coupling relationship between the shelf-edge trajectories and slope morphology. In general, concave-up slopes correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories and high-quality reservoirs developed on the basin floor under the influence of river-associated and shelf-incising canyons which have bright prospects for oil and gas exploration. Additionally, sigmoidal slopes usually correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories at times of relative sea-level fall and the reservoirs mostly developed on the upper slope under the influence of headless canyons. Moreover, the planar slopes correspond to high-angle ascending trend trajectories which are hardly potential for exploration. The coupling model built in this study will provide an insight for oil and gas exploration in deep-water areas with limited data and low exploration degree.

Morphological characteristics and evolution model of slope units along loess gully cross section

Gully morphology is an important part of loess landform research.Along with gully development,the variation of its cross section is the most significant,and it can intuitively reflect the characteristics of the lateral widening of the gully slope.Therefore,in-depth research of the variation of the cross-sectional morphology of the gully is helpful to understanding the development process of the loess gully.Based on the DEMs(Digital Elevation Model)of nine periods of an indoor simulated loess small watershed,this paper studies the evolution model of a complete branch ditch in the watershed using the digital terrain analysis theory and method.Results show that with the development of the gully,the average gradient of the gully slope continuously decreases,and the slope morphology is mostly a concave slope along the slope direction.The degree of downward concave first increases and then gradually tends to be gentle.The gully erosion mode is gradually transformed from downward cutting erosion to lateral erosion.The more mature the gully development,the lower the depth of gully bottom cutting is compared with the width of gully widening.Furthermore,the surface cutting depth tends to be stable and the slope is stable.Then,the transformation law of the slope morphology of the gully cross section with the development of the gully is studied,and the prediction model of the transformation of the slope morphology of the gully cross section is established by using the Markov chain.The Markov model can better reflect the dynamic change of the slope morphology of the gully cross section,which is of great significance to revealing the external performance and internal mechanism of the gully morphology.